R. Plume

Dense gas and star formation: characteristics of cloud cores associated with water masers

We have observed 150 regions of massive star formation, selected originally by the presence of an H2O maser, in the J = 5 → 4, 3 → 2, and 2 → 1 transitions of CS, and 49 regions in the same transitions of C34S. Over 90% of the 150 regions were detected in the J = 2 → 1 and 3 → 2 transitions of CS, and 75% were detected in the J = 5 → 4 transition. We have combined the data with the J = 7 → 6 data from our original (1992) survey to determine the density by analyzing the excitation of the rotational levels. Using large velocity gradient models, we have determined densities and column densities for 71 of these regions. The gas densities are very high (log n = 5.9), but much less than the critical density of the J = 7 → 6 line. Small maps of 25 of the sources in the J = 5 → 4 line yield a mean diameter of 1.0 pc. Several estimates of the mass of dense gas were made for the sources for which we had sufficient information. The mean virial mass is 3800 M☉. The mean ratio of bolometric luminosity to virial mass (L/M) is 190, about 50 times higher than estimates made using CO emission, suggesting that star formation is much more efficient in the dense gas probed in this study. The depletion time for the dense gas is ~1.3 × 107 yr, comparable to the timescale for gas dispersal around open clusters and OB associations. We find no statistically significant line width-size or density-size relationships in our data. Instead, both line width and density are greater for a given size than would be predicted by the usual relationships. We find that the line width increases with density, the opposite of what would be predicted by the usual arguments. We estimate that the luminosity of our Galaxy (excluding the inner 400 pc) in the CS J = 5 → 4 transition is 15-23 L☉, considerably less than the luminosity in this line within the central 100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to CS luminosity is higher in M82 than in any cloud in our sample.

The Submillimeter Wave Astronomy Satellite: Science Objectives and Instrument Description

The Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998 December, is a NASA mission dedicated to the study of star formation through direct measurements of (1) molecular cloud composition and chemistry, (2) the cooling mechanisms that facilitate cloud collapse, and (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense [n(H2) > 103 cm-3] molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: H2O, H218O, O2, C I, and 13CO. By observing these lines SWAS is (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS carries two independent Schottky barrier diode mixers—passively cooled to ~175 K—coupled to a 54 × 68 cm off-axis Cassegrain antenna with an aggregate surface error ~11 μm rms. During its baseline 3 yr mission, SWAS is observing giant and dark cloud cores with the goal of detecting or setting an upper limit on the water and molecular oxygen abundance of 3 × 10-6 (relative to H2). In addition, advantage is being taken of SWASs relatively large beam size of 33 × 45 at 553 GHz and 35 × 50 at 490 GHz to obtain large-area (~1° × 1°) maps of giant and dark clouds in the 13CO and C I lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H2O, O2, C I, and 13CO lines or the H218O, O2, and C I lines. All measurements are being conducted with a velocity resolution less than 1 km s-1.

The Ionization Fraction in Dense Molecular Gas. I. Low-Mass Cores

Observations of C18O, H13CO+, and DCO+ toward 23 low-mass cores are used to constrain the fractional ionization (electron abundance) within them. Chemical models have been run over a wide range of densities, cosmic-ray ionization rates, and elemental depletions, and we find that we can fit 20 of the 23 cores for densities of nH2=(1-3)×104 cm-3, moderate C and O abundance variations, and a cosmic-ray ionization rate of ζH2=5×10−17 s-1. The derived ionization fractions lie within the range 10-7.5 to 10-6.5, with a median value of xe,m = 9 × 10-8 and typical errors for each individual core equal to a factor of 3. These values imply that the cores are weakly coupled to the magnetic field and that MHD waves can propagate within them. The ambipolar diffusion timescale is about an order of magnitude greater than the free-fall time, and the cores can be considered to be in quasi-static equilibrium. There is no significant difference between the ionization fraction for cores with and without embedded stars, which suggests that the molecular ionization in cores is primarily governed by cosmic rays alone.

Implications of Submillimeter Wave Astronomy Satellite Observations for Interstellar Chemistry and Star Formation

A long-standing prediction of steady state gas-phase chemical theory is that H2O and O2 are important reservoirs of elemental oxygen and major coolants of the interstellar medium. Analysis of Submillimeter Wave Astronomy Satellite (SWAS) observations has set sensitive upper limits on the abundance of O2 and has provided H2O abundances toward a variety of star-forming regions. Based on these results, we show that gaseous H2O and O2 are not dominant carriers of elemental oxygen in molecular clouds. Instead, the available oxygen is presumably frozen on dust grains in the form of molecular ices, with a significant portion potentially remaining in atomic form, along with CO, in the gas phase. H2O and O2 are also not significant coolants for quiescent molecular gas. In the case of H2O, a number of known chemical processes can locally elevate its abundance in regions with enhanced temperatures, such as warm regions surrounding young stars or in hot shocked gas. Thus, water can be a locally important coolant. The new information provided by SWAS, when combined with recent results from the Infrared Space Observatory, also provides several hard observational constraints for theoretical models of the chemistry in molecular clouds, and we discuss various models that satisfy these conditions.

Water abundance in molecular cloud cores

We present Submillimeter Wave Astronomy Satellite (SWAS) observations of the 110 → 101 transition of ortho-H2O at 557 GHz toward 12 molecular cloud cores. The water emission was detected in NGC 7538, ρ Oph A, NGC 2024, CRL 2591, W3, W3OH, Mon R2, and W33 and was not detected in TMC-1, L134N, and B335. We also present a small map of the H2O emission in S140. Observations of the H218O line were obtained toward S140 and NGC 7538, but no emission was detected. The abundance of ortho-H2O relative to H2 in the giant molecular cloud cores was found to vary between 6 × 10-10 and 1 × 10-8. Five of the cloud cores in our sample have previous H2O detections; however, in all cases the emission is thought to arise from hot cores with small angular extents. The H2O abundance estimated for the hot core gas is at least 100 times larger than in the gas probed by SWAS. The most stringent upper limit on the ortho-H2O abundance in dark clouds is provided in TMC-1, where the 3 σ upper limit on the ortho-H2O fractional abundance is 7 × 10-8.

A survey of CS J = 7 - 6 in regions of massive star formation

The results of a survey for CS J=7→6 emission towards 179 star-forming regions, selected by the presence of an H 2 O maser with an accurate position, are presented. The line was detected in 104 sources (58%); the detections have a mean T * R of 2.9 K and a mean ∫T * R dv of 28 K km/s. Using LVG excitation calculations, the densities are estimated between log (n) of 4.7 and 6.7 for a CS 7→6 optical depth of 1

The ionization fraction in dense molecular gas. II. Massive cores

We present an observational and theoretical study of the ionization fraction in several massive cores located in regions that are currently forming stellar clusters. Maps of the emission from the J=1→0 transitions of C18O, DCO+, N2H+, and H13CO+, as well as the J=2→1 and 3→2 transitions of CS, were obtained for each core. Core densities are determined via a large velocity gradient analysis with values typically of ~105 cm-3. With the use of observations to constrain variables in the chemical calculations, we derive electron fractions for our overall sample of five cores directly associated with star formation and two apparently starless cores. The electron abundances are found to lie within a small range, -6.9 < log10xe < -7.3, and are consistent with previous work. We find no difference in the amount of ionization fraction between cores with and without associated star formation activity, nor is any difference found in electron abundances between the edge and center of the emission region. Thus our models are in agreement with the standard picture of cosmic rays as the primary source of ionization for molecular ions. With the addition of previously determined electron abundances for low-mass cores, and even more massive cores associated with O and B clusters, we systematically examine the ionization fraction as a function of star formation activity. This analysis demonstrates that the most massive sources stand out as having the lowest electron abundances (xe < 10-8).

Observations of Water Vapor toward Orion BN/KL

We have obtained spectra of the rotational ground-state 110-101 556.936 GHz ortho-H216O and 110-101 547.676 GHz ortho-H218O transitions toward Orion BN/KL using the Submillimeter Wave Astronomy Satellite (SWAS). The ortho-H216O spectrum shows strong evidence for both a broad (Δv 48 km s-1) and a narrow (Δv 7.5 km s-1) component, while the ortho-H218O shows evidence for only a broad (Δv 24 km s-1) component. The broad component emission in both ortho-H216O and ortho-H218O arises primarily from gas heated within the low- and high-velocity outflows and shocked gas surrounding IRc2 in which the ortho-H216O and ortho-H218O fractional abundances are estimated to be 3.5 × 10-4 and 7 × 10-7, respectively. This finding provides further confirmation that water is efficiently and abundantly produced within warm shock-heated gas. We estimate that the hot core plus the compact ridge contribute 10% to the ortho-H216O integrated intensity within the SWAS beam. The narrow component seen in the ortho-H216O spectrum is best fitted by ortho-water emission from the extended ridge (ER) and the higher temperature core of the extended ridge (CER) with a common fractional abundance of 3.3 × 10-8. The absence of any discernible narrow component in the ortho-H218O spectrum is used to set 3 σ upper limits on the ortho-water fractional abundance within the ER of 7 × 10-8 and within the CER of 5.2 × 10-7. This implies that within the dense extended quiescent region, gas-phase water is neither a major repository of oxygen nor a major coolant in Orion BN/KL.

Submillimeter Wave Astronomy Satellite Observations of Jupiter and Saturn:Detection of 557 GHz Water Emission from the Upper Atmosphere

We have used the Submillimeter Wave Astronomy Satellite to carry out observations on Jupiter and Saturn in two bands centered at 489 and 553 GHz. We detect spectrally resolved 557 GHz H2O emission on both planets, constraining for the first time the residence levels of external water vapor in Jupiters and Saturns stratosphere. For both planets, the line appears to be formed at maximum pressures of about 5 mbar. For Jupiter, the data further show that water is not uniformly mixed but increases with altitude above the condensation level. In each planet, the amount of water implied by the data is 1.5-2.5 times larger than inferred from Infrared Space Observatory data. In addition, our observations provide new whole-disk brightness measurements of Jupiter and Saturn near 489 and 553 GHz.

Observations of Absorption by Water Vapor toward Sagittarius B2

We have observed the 110-101 pure rotational transitions of both H216O and H218O toward Sagittarius B2 using the Submillimeter Wave Astronomy Satellite. The spectra thereby obtained show a complex pattern of absorption and—in the case of H216O—emission, with numerous features covering a wide range of LSR velocities (-130 to 130 km s-1) and representing absorption both in gas associated with Sgr B2 as well as by several components of foreground gas along the line of sight. The ortho-water abundance derived for the absorbing foreground gas is ~6 × 10-7 relative to H2.